1. Field of the Invention
The present invention relates to a magnetic resonance imaging system and, more particularly, to a magnetic resonance imaging system which permits acquisition of a magnetic resonance image of an object under examination.
2. Description of the Related Art
As is well known, magnetic resonance imaging (MRI) is a method of imaging chemical and physical information of molecules utilizing a magnetic resonance phenomenon in which, when placed in a uniform static magnetic field of an intensity of Ho, nuclear spins having an intrinsic magnetic moment absorb the energy of a radio-frequency magnetic field rotating at a specific angular velocity .omega.=.gamma. Ho (.gamma.=gyromagnetic ratio) in a plane orthogonal to the direction of the static magnetic field.
Methods of imaging spatial distribution of specific nuclei (for example, hydrogen atomic nuclei, i.e., proton, within water or fat) within an object under examination using the magnetic resonance imaging technique include the projection reconstruction method by Lauterbur, the Fourier method by Kumar, Welti or Ernst, the spin warp method by Hutchison which is a modification of the Fourier method and so on.
With magnetic resonance imaging systems for obtaining images by the use of the magnetic resonance imaging, it takes a long time to acquire data as compared with other medical image diagnostic apparatuses such as ultrasonic diagnostic apparatus and X-ray computed tomography apparatus. Hence, artifacts may be produced by movement of an object under examination such as physiological motion due to respiration. A problem thus arises in that imaging of parts of the object which move rapidly, such as his or her heart and blood will cause the subject a great deal of pain.
As methods of obtaining images at high speed by the magnetic resonance imaging, therefore, ultra high speed imaging methods have been proposed which include the echo planar method by Mansfield and the ultra high speed Fourier method by Hutchison.
With the ultra high speed Fourier method, magnetic resonance data are acquired in accordance with such a pulse sequence as shown in FIG. 1.
(1) A 90.degree. radio frequency selective excitation pulse adapted to create a radio frequency field RF is applied to a selected slice of the body of an object under examination subjected to a gradient field Gs for slice selection to selectively excite spins within the slice.
(2) A 180.degree. radio frequency pulse is applied to the slice for the excitation of a spin echo.
(3) With a readout gradient field Gr applied in the direction parallel to the slice plane while being switched in polarity a plural number of times at high speed, a phase-encoding gradient field Ge is applied, in the direction which is parallel to the slice plane an orthogonal to the readout gradient field Gr, to the slice in a predetermined polarity before the repeated switching of the polarity of the readout gradient field Gr and then applied in a pulse form at each switch of the polarity of the readout gradient field Gr with the polarity of the field Ge reversed. The interval when the readout gradient field Gr is applied with its polarity switched is a data observation interval Tobs.
The sequence of FIG. 1 is for the so-called full encode method by which, as illustrated in FIG. 2 which shows the manner of data acquisition on Fourier space (k-space), sequential encoding is performed to acquire all the data in phase space, or Fourier space utilizing information on both sides (before and after) of the center 2.tau. of the spin echo.
As shown in FIG. 3, the ultra high speed Fourier method utilizing the so-called half-encoding method is different from the full-encoding method of FIG. 1 in that the readout gradient field Gr is reversed repeatedly only on one side of the spin echo center 2.tau.' and the phase-encoding gradient field Ge is not reversed before the repeated switching of the readout gradient field Gr. In this case, though only data on the half plane of the Fourier space are acquired, data on the other half plane can be obtained by taking the complex conjugate of the acquired magnetic resonance data. For example, if the complex conjugate of data S(t) on the Fourier space is taken, then data S(-t)=S*(t) will be obtained. Here the reason why data are collected only on one side of the spin echo center 2.tau.' is that, because data of a half plane are produced by taking the complex conjugate it is desired to collect central data of phase space at the spin echo center in which the inhomogeneity of the magnetic field is minimum.
With the echo planar method, on the other hand, the encoding gradient field Ge is applied statically and continuously during the interval in which the readout gradient field is switched repeatedly instead of being applied intermittently as shown in FIGS. 1 and 3. In this case, on the Fourier space, data acquisition is performed not in parallel with axis kx corresponding to the readout direction as in the cases of FIGS. 2 and 4 but in a zigzag.
According to those methods, image data of a slice can be acquired during a period of time the magnetization of spins in the slice excited by the 90.degree. radio frequency pulse relaxes because of the relaxation phenomenon of transverse magnetization, thus permitting ultra high speed imaging.
However, the above ultra high speed imaging methods have the following problems.
Phase dispersion occurs with the excited spins within a slice because of spatial inhomogeneity of the static field and apparent transverse relaxation occurs during time T2* less than the transverse relaxation time T2. Accordingly, correct image reconstruction cannot be achieved because of the phase dispersion due to the influence of the inhomogeneity when the period of the data acquisition or the signal observation is long in comparison with the apparent transverse relaxation time T2. With pulse sequences for the ultra high speed imaging shown in FIGS. 1 and 3, after excitation of spins by a 90.degree. selective excitation pulse, a 180.degree. radio frequency pulse is applied to rephase the spins which have been dephased so that magnetic resonance data may be acquired. It should be noted that the data acquisition is not performed immediately after the application of the 90.degree. selective excitation pulse. According to this approach, the 180.degree. pulse is applied after a time of .tau. or .tau.' from when the 90.degree. pulse is applied and the spins are rephased after a time of .tau. or .tau.' from when the 180.degree. pulse is applied (this point corresponds to the spin echo center 2.tau. or 2.tau.'). Thus, the observation time Tobs is set in the neighborhood of the time at which the spins are rephased. Artifacts will be produced when physiological motion of a subject under examination occurs during the observation interval Tobs. Hence, all the magnetic resonance data required for obtaining an image must be collected within a short length of time (for example, about 30 milliseconds) during which the spins have been rephased and the physiological motion of the subject has little influence.
In the above echo planar method and ultra high speed Fourier method, the gradient fields are switched at high speed to sample magnetic resonance data. However, the number of gradient echoes which are generated or available during such a short length of time as described above, namely, the number of encode steps has a limit because of limitations of switching speed of the gradient fields (for example, rising time of about 300 .mu.s) and sampling speed. For example, in the case of FIG. 1 where both sides of the spin echo center 2.tau. are utilized, the number of the gradient echoes is limited to about 64. Thus, high resolution in the encode direction is difficult to achieve. Where the half encode method is utilized, the high resolution is still difficult to achieve because data are collected on one side of the spin echo as described above.